Carbon is one of the important elements in cast iron. It reduces melting point of iron. Pure iron has a melting point of about 1500°C but iron with 3.50% C has melting point of about 1350°C. When carbon is in free form i.e. as graphite form, the resulting cast iron is known grey cast iron. On the other hand, when the iron and carbon are chemically combined form of cementite, the cast iron will be hard and known as white cast iron.
Presence of silicon in cast iron promotes the decomposition of cementite into graphite. It also helps to reduce the shrinkage in cast iron when carbon is changed to graphite forms.
It makes the cast iron hard and brittle. Since too much sulphur gives unsound casting, therefore, it should be kept below 0.1% for most casting purposes. It is often responsible for creating troubles to foundry men. It will make cast iron hard thereby counteracting the softening influences of silicon. It decreases strength and increases brittleness. It also promotes oxidation of cast iron. Hence, it is kept as low as possible in cast iron.
It makes cast iron white and hard. It is often kept below 0.75%. It helps to exert a controlling influence over the harmful effect of sulphur. It reduces the harmful effects of the sulphur by forming the manganese sulphide which is not soluble in cast iron.
It increases fusibility and fluidity in cast iron but induces brittleness. It is rarely allowed to exceed 1%. Phosphorus in irons is useful for casting of intricate shapes and for producing very cheap and light engineering castings. Phosphorus has no effect on the carbon as well as on shrinkage in the cast iron.